1. Field of the Invention
The invention generally relates to battery charge control circuits and, more particularly, to a charge control circuit for an auxiliary battery charged by a primary vehicle battery.
2. Brief Description of Related Technology
Charge control circuits for regulating the charging of Nickel-Cadmium (NiCd), Nickel-Metal-Hydride (NiMH), and other rechargeable batteries from a DC source are well known. In fact, a portion of the circuitry involved in such circuits is made available commercially as a discrete integrated circuit (IC). Such ICs are often referred to as charge controllers. One commercially available charge controller IC is a pin-programmable, fast-charge controller from Maxim Integrated Products under product number MAX712 or MAX713 (www.maxim-IC.com, Sunnyvale, Calif.).
Circuits for charging batteries often employ a control or feedback scheme based on an evaluation of the voltage of the battery cell(s) being charged. For example, a battery charging circuit may facilitate low-current charging, or trickle charging, until the battery cell being charged reaches a threshold voltage. Other characteristics of the battery cell(s) being charged, such as charging current and battery temperature, have also been evaluated to control charging.
In many cases, these charge control circuits are applied in the context of a power source of effectively infinite capacity. The control circuit is thus designed without regard to whether the power source could be adversely affected by the charging operation. However, in cases where one battery is charging another, the discharging of the source battery may need to be regulated.
The discharging of a vehicle battery has been regulated to ensure sufficient capacity for engine start via, for example, the operation of a starter motor. Complicating matters somewhat, vehicle batteries exhibit large voltage swings based on whether the engine is running or, more specifically, whether an alternator is operating to convert mechanical power to electrical power. For instance, a typical 12-volt, automobile battery may be at approximately 13.8 Volts with the alternator operating.
Without the power supplied by the alternator, the voltage of the vehicle battery drops noticeably with the use of accessories requiring significant power or current. Once the vehicle battery drops below a threshold voltage, past discharge regulators have prohibited certain accessory use. But despite such voltage drops, the vehicle battery may nevertheless have sufficient capacity to power certain, low-power accessories, particularly if the power consumption of such accessories could be regulated in response to the condition of the vehicle battery. In this manner, power consumption by a vehicle accessory may be permissible well after the engine has stopped running and the voltage of the vehicle battery has decreased.